1. Field of the Invention
The present invention relates to an improvement in an uninterruptible power supply for preventing malfunction or stop of a computer by occurrence of momentary interruption, and especially relates to a flywheel type energy storage apparatus used in the uninterruptible power supply, for storing kinetic energy, so that the kinetic energy is converted to electric energy for supply for a computer when the momentary interruption occurs.
2. Description of the Prior Art
A flywheel type energy storage apparatus to be used in an uninterruptible power supply stores energy in the form of kinetic energy by converting the electric energy to kinetic energy of a revolving flywheel, which rotates at a high speed and re-converts the kinetic energy to electric energy for supplying electric power to a load in case of a necessity, such as occurrence of a momentary interruption.
A conventional flywheel type energy storage apparatus, for example, shown in Japanese Unexamined Patent Publication Sho 52-65804 is described referring to FIG. 2. In FIG. 2, a flywheel 2 is rotatably born in a vacuum cavity 1. The flywheel 2 is made of a magnetic material, formed in a disc shape and disposed in a manner that its axis is parallel to the force of gravity. Ring-shaped members 3a and 3b are formed to have their sections protruding upward and downward from the upper and the lower faces of the disc. The upper and the lower shaft members 4a and 4b are fixed coaxially to the centers of the upper and the lower faces of the flywheel 2. The lower end of the lower shaft member 4b is born by a spherical bearing 5 fixed on the bottom of the vacuum chamber 1.
Furthermore, a magnetic bearing 6 is provided in the vicinity of the ring-shaped members 3a and 3b. The magnetic bearing 6 comprises a ring-shaped magnetic frame 7 which is disposed in a manner that pole faces 7a and 7b thereof face top and bottom faces of the ring-shaped members 3a and 3b with predetermined gap and a excitation coil 8 mounted in the magnetic frame 7. Hereupon, a gap P between the upper ring-shaped member 3a and the upper pole face 7a of the magnetic frame 7 is set to be narrower than a gap Q between the lower ring-shaped member 3b and the lower pole face 7b of the magnetic frame 7.
On the other hand, a rotor 10 of a main generator 9 which serves as a motor is fixed on outer periphery of the lower shaft member 4b. And a stator 11 of the main generator 9 is fixed a position to magnetically couple the rotor 10. Furthermore, a rotor 13 of sub-generator 12 is fixed to the upper shaft member 4a, and a stator 14 of the sub-generator 12 is fixed at a position to magnetically couple the rotor 13.
Lead wires of windings of the rotors 10, 13 and the stators 11, 14 of the generators 9 and 12 and the excitation coil 8 of the magnetic bearing 6 are led out of the vacuum chamber 1 through airtight sealings. Interior of the vacuum chamber 1 is maintained in sufficiently low pressure.
The excitation coil 8 of the magnetic bearing 6 is normally excited. When the excitation coil 8 is excited, magnetic flux induced by the excitation coil 8 passes a closed magnetic circuit connecting of the magnetic frame 7 and the ring-shaped members 3a and 3b. Therefore, thrust magnetic attraction occurs between the pole faces of the magnetic frame 7 and the ring-shaped members 3a and 3b. As a result, thrust load of the flywheel 2 is created.
Since the gap P is set to be narrower than the gap Q, a magnetic attraction force in an upward direction in FIG. 2 acts on the flywheel 2. Accordingly, a thrust load actually applied on the spherical bearing 5 becomes only a fraction of weight of the flywheel.
In the above-mentioned conventional flywheel type energy storage apparatus, the main generator 9 is driven as a motor, and the flywheel starts to rotate by electrification of the excitation coil 8. When the rotation speed of the flywheel 2 reaches a predetermined value, the main generator 9 is switched to a generator. At that time, kinetic energy stored in the flywheel 2 is converted to electric energy and returned to the power line as electric power.
Generally, the amount of the kinetic energy stored in the flywheel is determined by the amount of inertia of the flywheel and the rotation speed thereof. Accordingly, it is necessary to increase the moment of inertia of the flywheel and the rotation speed thereof for increasing the amount of the kinetic energy stored in the flywheel.
In the conventional flywheel type energy storage apparatus shown in FIG. 2, the ring-shaped members 3a and 3b are formed integrally on peripheral parts of the front and rear faces of the flywheel 2. In such configuration, it is impossible to obtain sufficiently high strength to withstand a very high rotation speed of the flywheel 2, which is suitable to store a large amount of kinetic energy, since a very large moment of the inertia of the flywheel is made by centrifugal force in the high speed rotation. Accordingly, it has been necessary to increase the diameter of the flywheel 2 or to make the ring-shaped members 3a and 3b very thick in the axial direction of the flywheel 2, to increase the amount of the kinetic energy stored in the flywheel 2. As a result, the conventional flywheel type energy storage apparatus has a disadvantage that the apparatus becomes very large and very heavy in order to store a large amount of kinetic energy.